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FAQ: 2020 Western Monarch Overwintering Numbers

Jan 22, 2021


  • Population Trends

On Tuesday, January 19, 2021 the Xerces Society shared their final results from the 2020 Western Monarch Thanksgiving Count. Across 261 overwintering sites, volunteers counted only 1,914 monarch butterflies. Read the full media release from Xerces, or a summary of the results in a previous MJV blog post.

This is distressing news and has many people asking good questions about what these numbers mean and what they can do to help. We’ve compiled a list of some of these questions below. The Xerces Society also released a blog post, which answers some of these questions in more detail.  As more information becomes available, Monarch Joint Venture will continue to share resources focused on western monarch butterflies and what you can do to support conservation efforts. Please follow us on social media (we’re on Facebook, Instagram, and Twitter) and subscribe to our news updates mailing list to stay up to date.

UPDATE: On Western Monarch Day, the Monarch Joint Venture launched the Western Monarch Recovery Fund. This fund will be used to immediately support shovel-ready projects in the West that will make a difference for the western migration. 


How do this year’s numbers compare to previous years?

The total count of 1,914 monarchs represents a 99.9% decline in the population of migrating western monarchs since the 1980s. California overwintering groves were once covered in an estimated 4.5 million monarchs each winter. When the Western Monarch Thanksgiving Count started in 1997, over 1.2 million overwintering monarchs were observed at 101 sites. In 2018 and 2019, the population of western monarchs was hovering at around 30,000 monarchs, which was the posited extinction threshold. This means that researchers predicted that monarch butterflies may not be able to sustain their population if it dropped below 30,000 butterflies.


Did the COVID-19 pandemic affect the ability of volunteers to count butterflies?

Even while implementing safety precautions amid the COVID-19 pandemic, volunteers were able to monitor a total of 246 sites in 2020, which is three more sites than were monitoring in 2019. This means that the drop in numbers is likely not due to human limitations from the pandemic. On average, the number of sites surveyed has increased, while the number of monarchs observed at overwintering sites has decreased over the last 24 years.


Where can I find more information about western monarch population trends?

The Western Monarch Thanksgiving Count has population records going back to 1997, which can be downloaded as an excel or pdf file. They also have a graph which shows the population trends of the same time period, contrasted with the number of sites surveyed.


Why is tropical milkweed a challenge for monarchs?

In short, tropical milkweed is a concern because in areas of the southern U.S. (such as areas along the Gulf coast and southern California) it usually does not die back in the winter like most native milkweeds. The continued presence of milkweed during the winter months can disrupt migration and encourage breeding during months when monarchs should be in reproductive diapause at their overwintering grounds. Reproductive diapause allows migrating monarchs to delay their reproduction which extends their life from a few weeks to a number of months, allowing them to complete the migration and survive the overwintering period. Climate variables are also thought to influence this winter breeding behavior.

In addition to disruptions to the migration itself, the persistence of milkweed year-round allows the protozoan parasite known as Ophryocystis elektroscirrha (OE) to build up on the plants and increase spread to monarch offspring that consume those plants. In contrast, native milkweeds that die back are able to “cleanse themselves” of the OE spores that build up on their leaves. This parasite can weaken monarchs, cause deformed wings, and is generally bad for their health. Allowing the prevalence of the parasite to increase in the migratory population also has detrimental effects on the overall population and their ability to migrate successfully. We already have evidence of this from the year-round, non-migratory population of monarchs in southern Florida, which is highly infected with the parasite. You can learn more about tropical milkweed and O.E. in this MJV handout.

We recommend replacing tropical milkweed in your yard with native milkweed and/or native nectar plants as you are able to. For areas directly adjacent to overwintering sites, native nectar plants are actually more important than native milkweed. More information about native milkweeds by region can be found on MJV’s milkweed handout and nectar plants native to your area can be found using the Xerces Monarch Nectar Plant Guides or Pollinator Partnership’s Eco Regional Guides.


If the overwintering numbers in California are so low, why am I seeing so many monarch butterflies in my yard?

There is anecdotal evidence to suggest that there may be an increase in the number of winter breeding monarchs in California, although this question does need to be researched more fully. If you are seeing monarchs, it may be a result of increased tropical milkweed or other environmental factors. To help researchers better understand these trends, please report your monarch observations to community science projects like Western Monarch Milkweed Mapper or Journey North. If you are able to document this behavior at the same site(s) on a weekly basis, we strongly encourage getting involved in the Monarch Larva Monitoring Project to keep a more detailed account of your observations! The previous question explores the challenges and unanswered questions of increased winter breeding activity.


I raise monarchs in the West. Is this helping the population?

IMPORTANT NOTE:  Handling monarchs, including rearing them, is against the law in the state of California. A Scientific Collections Permit (SCP) is required to handle or rear monarchs since they are listed on the California Terrestrial and Vernal Pool Invertebrates of Conservation Priority List. Learn more here.

There is no evidence to suggest that raising monarch butterflies increases population numbers and it is not an effective conservation strategy. To the contrary, raising more than just a few monarch butterflies each summer for enjoyment, community science, or educational purposes may cause more harm than good. Just like with tropical milkweed, rearing monarchs together can increase the spread of OE in monarchs, as well as many other viral or bacterial pathogens. MJV created this rearing handout that goes over the whys and why nots of rearing, and provides our recommendations. If you choose to rear a few monarchs for educational purposes or personal enjoyment, we encourage you to provide data to community science projects like the Monarch Larva Monitoring Project, and to test for OE through Project Monarch Health.


What can I do to help?

We need everyone to get involved to support western monarchs! Here are a few opportunities to get started:

  1. Create pollinator habitat by planting native milkweed and nectar plants. Overwintering monarchs will begin leaving as early as February, so if you are in more temperate areas of the West, begin planting as soon as possible.
  2. Report your observations to monarch community science projects. If you are in the West, two great programs are Western Monarch Milkweed Mapper or Journey North. More detailed weekly observations are strongly encouraged if possible, which should be reported through the Monarch Larva Monitoring Project.
  3. Work to protect overwintering habitat in California. 
  4. Spread awareness about monarch declines and conservation opportunities.
  5. Support organizations working to protect western monarchs. 
  6. Learn more in the Western Monarch Call to Action and follow MJV on social media for updates as more resources and projects become available to support western monarchs.


Is there legislative action being taken to protect the monarch?

In December, the US Fish and Wildlife Service announced that listing the monarch butterfly under the Endangered Species Act is warranted but precluded. This means that while they do not have any additional protections at this time, monarchs are a candidate species and will be re-evaluated for listing on an annual basis. In the meantime, we need to step up voluntary conservation efforts.

One program that you can pursue now is the Mayors’ Monarch Pledge through the National Wildlife Federation. By signing the pledge by March 31st, U.S. cities, municipalities, and other communities commit to take at least three actions this year that support monarchs and pollinators. If you are based in California, one suggested action is passing a resolution to protect over-wintering monarch butterfly habitat on public and private lands. This page provides information on how to get your local community and legislators involved

We are also working with some of our partners to explore additional collective action initiatives and will provide more information and resources as they become available.


The Monarch Joint Venture is a 501c3 nonprofit organization and a national partnership of federal and state agencies, non-governmental organizations, businesses and academic programs working together to conserve the monarch butterfly migration. The content in this article does not necessarily reflect the positions of all Monarch Joint Venture partners. Header photo is of overwintering monarchs in California taken in 2013 by Wendy Caldwell.